The measurement of cardiac output, i.e., the total volume of blood pumped by the heart per unit of time provides important information about the effectiveness of the heart as a pump and of blood circulation. Many techniques, such as indicator dilution and ultrasound techniques, have been employed to obtain cardiac output.
Thermodilution is one common form of indicator dilution used to obtain cardiac output. With this technique, a thermodilution catheter is placed in the right heart so that an injection port of the catheter is in the right atrium and a thermistor on the catheter is located downstream in the pulmonary artery. A bolus of cold saline is injected into the right atrium through the injection port where it mixes with the blood and produces a temperature change which is detected by the thermistor. From this, a thermodilution curve can be plotted, and the shape of the curve depends on the flow rate.
One problem with a thermodilution technique that involves the injection of cold saline is that it cannot be carried out continuously. Accordingly, this technique is not suitable for providing continuous cardiac output information.
There is a natural variation in temperature in the blood at certain upstream and downstream locations in the circulatory system. For example, the temperature at upstream locations, such as the inferior vena cava, the superior vena cava and the right atrium, varies as does the temperature downstream in the pulmonary artery. The blood temperature at these locations varies with respiration, and the temperature variations at the upstream locations are out of phase with the variations at the downstream location. By appropriately processing this temperature information, the length of time required for the blood to travel between the upstream and downstream locations and the blood velocity between these two locations can be determined. Although techniques for accomplishing this are known, this velocity information is not usable directly to determine cardiac output because, over a length of time, the volume of the system between the two locations at which the temperapure is measured may vary due to, for example, medication taken by the patient and variations in blood volume.
Ultrasound techniques can be used to obtain continuous cardiac output. However, the ultrasound equipment is expensive and it requires the continuous presence of a skilled technician to assure that the ultrasound equipment provides accurate results.